The present invention relates initially, and thus generally, to flow straightening techniques and apparatus. More specifically, the present invention relates to techniques and apparatus for preventing the formation of cylindrical vortices in applications where they are detrimental to performance. The applications include, but are in no way limited to, vortex attractors (as disclosed in inventor""s co-pending application Ser. No. 09/316,318 entitled xe2x80x9cVortex Attractorxe2x80x9d, which is herein incorporated by reference), VTOL aircraft and lifting platforms (an example of which is disclosed in inventor""s co-pending Ser. No. 09/728,602, entitled xe2x80x9cLifting Platformxe2x80x9d, which is herein incorporated by reference), toroidal vortex attractors (as disclosed in inventor""s co-pending Ser. No. 09/829,416, entitled xe2x80x9cToroidal and Compound Vortex Attractorxe2x80x9d, which is herein incorporated by reference), and toroidal vortex vacuum cleaners (as disclosed in inventor""s co-pending application Ser. No. 09/835,084, entitled xe2x80x9cToroidal Vortex Bagless Vacuum Cleanerxe2x80x9d, which is herein incorporated by reference).
The need for flow straightening techniques and apparatus has been well established in the prior art, as well as what has been disclosed in prior applications of the inventor. For example, the prior art has prescribed the use of flow straightening in numerous applications such as gas scrubbers, helicopter rotors and fluid flow control. The inventor has specifically prescribed their use in applications such as toroidal vortex attractors, lifting platforms and vacuum cleaners. For the purposes of understanding the fundamental flow characteristics that make flow straightening useful, the flows of very basic systems will first be examined.
FIG. 1 shows a top view of a common prior art horizontal propeller. The construction consists of a motor 102 coupled to a plurality of blades 100 (in this example, four blades are shown). The blades rotate in accordance with direction 101. FIG. 2 shows a side view of the same system consisting of blades 100 and motor 102. In this view, however, the vertical airflow components 200 are shown, with air drawn downwards through the blades 100 and outwards at the ends of the blades 100. Air drawn downwards through the blades has a pressure, initially lower than ambient, that can be determined using Bernoulli. The outward curvature, however, leads to pressure higher than that above the blades 100. The pressure difference across a curved stream tube is defined by the equation p=(xcfx81*V2)/R, where xe2x80x9cpxe2x80x9d is the pressure difference, xe2x80x9cxcfx81xe2x80x9d is the air density, xe2x80x9cVxe2x80x9d is the airspeed and xe2x80x9cRxe2x80x9d is the radius of curvature of the stream tube. The increased pressure beneath the blades adds to the efficiency of the propeller and lift. As seen, the curved airflow 200 forms a partial toroidal vortex 201 and the tips of blades 100.
Of particular concern, however, is the deflection of downward airflow through the blades in the direction of blade movement. This effect is illustrated in FIG. 3. As blades 100 rotate (the direction of rotation is shown by arrow 300), momentum is imparted to the incoming airflow 301 such that it is deflected as shown.
FIG. 4 shows a top view of the air flow 301 of FIG. 3. Again, motor 102 coupled to propeller blades 100. Specifically, the outward radial airflow 400 combines with tangential flow 401 due to the blade movement to form a resultant spiraling air flow 402. The effect is more pronounced when the blade 100 pitch is high.
The radial air movement 400 produces a cylindrical vortex. Such vortices have been exhaustively described by the inventor in prior applications. The cylindrical vortex results in a lowering of the air pressure beneath the propeller that is approximately p=(xcfx81*Vt2)/Rt, where xe2x80x9cpxe2x80x9d is the air pressure, xe2x80x9cVtxe2x80x9d is the tangential air velocity at the tip of propeller 100 produced by its rotation, xe2x80x9cRtxe2x80x9d is the propeller tip radius and xe2x80x9cxcfx81xe2x80x9d is the air density. Consequently, there are two conflicting pressure generation systems. The first (the partial toroidal vortex), provides increased pressure and lift, while the second (the cylindrical vortex) leads to decreased pressure and decreased lift.
When very close to the ground and when operating with a high propeller pitch, the cylindrical vortex may predominate leading to a critical loss of lift. Thus, a helicopter, or VTOL (vertical take-off and landing) aircraft when very close to the ground may encounter conditions in which the cylindrical vortex predominates and lift is lost. An attempt to counter this by increasing the propeller pitch will increase the cylindrical vortex and further reduce the pressure beneath the blades and thereby lead to a crash. The inventor theorizes that this effect could solve unexplained crashes of helicopters and VTOL aircraft such as the Osprey.
FIG. 5 shows the addition of flow straightening vanes 500 beneath propeller 100 such that the air that enters 502 the propeller-vane system exits 503 with a vertical motion. However, such a system is cumbersome and could not be practically added to a helicopter. Alternatively, such flow straightening vanes have been integrated into prior systems of the inventor""s that utilize an outer shroud or a duct.
Thus, it is clear that in order to properly remove rotational components from a flow, flow straightening techniques are necessary and useful. However, while flow straightening vanes have been useful in prior systems (such systems have been the subject of several of the inventor""s co-pending patent applications, several of which have already been incorporated by reference), such systems could benefit from flow straightening techniques in accordance with contra-rotating blades, propellers, or impellers.
While the prior art does disclose the use of contra-rotating propellers, no where has their use been disclosed as a means to eliminate cylindrical vortices to increase performance of devices employing toroidal vortices. However, the following represent references that the inventor feels are the most relevant, and still, do not approach the scope of the present invention.
Smith et al U.S. Pat. No. 4,422,342 is directed to a method for flow straightening and sampling gas in the stack of a gas scrubber. In normal operation, the gas has a substantially non-axial flow so as to retain, within the stack, droplets carried by the gas. A flow straightener having flow straightening panels is placed in the chamber of the gas scrubber and lifted into the stack. Gas having an axial flow is then sampled. The flow straightener is then removed from the scrubber, and normal operation is resumed. Smith et al simply provides for removable flow straightening vanes to allow for more accurate sampling of an effluent. Nowhere does Smith et al suggest the use of contra-rotating propellers or the elimination of cylindrical vortices.
Marze et al U.S. Pat. No. 5,634,611 discloses an arrangement for a helicopter rotor in which a flow-straightening stator is fixed into the duct downstream of the rotor. It is mounted so that it rotates in the duct, and includes vanes with an aerodynamic profile that straightens out the airflow downstream of the rotor substantially along the axis of the duct, and are each inclined to the radial direction, from the axis of the duct towards its periphery, and in the opposite direction from the direction of rotation of the rotor, and/or inclined at a slant, from the center of the duct to its periphery and from upstream to downstream of the duct. This arrangement is said by the inventors to increase safety, increase performance and decrease noise. However, Marze et al do not disclose any new method or apparatus for straightening a flow, just a particular arrangement therefor. Moreover, since Marze et al are not concerned in this reference with the lift generation of a helicopter, they cannot appreciate the detrimental effects of a cylindrical vortex, and therefore, cannot contemplate any need for its removal.
Larkin U.S. Pat. No. 5,795,200 teaches a marine propulsion unit having two contra-rotating propellers driven by a single shaft. The propellers are driven through gearing which includes a set of static planetary gears, an external gear, and an internal gear. The gearing is mounted in an oil-filled gear box having three co-axial cylindrical sections. The outer two sections rotate in opposite directions and drive propeller hubs. The intermediate section is stationary and serves as a carrier for the planetary gears and a mount for flow-straightening vanes which support a cylindrical shroud. An angular passage may be provided in the intermediate section to exhaust combustion gases through openings in the outside of the shroud when the unit forms part of an outboard motor. Larkin does disclose contra-rotating propellers, but does not employ them for the explicit purpose of straightening flow since a separate flow-straightening stage between the two propellers is prescribed. Furthermore, nowhere does Larkin appreciate the effects of a vortex flow.
Crockett U.S. Pat. No. 5,816,907 teaches a vehicle air outlet having an outlet bell, an upstream shutoff valve having an open and closed position and a direction control member which is downstream of the shutoff valve. The upstream shutoff valve includes a flow straightener assembly that is functional when the shutoff valve is in a fully opened position and non-functional when the shutoff valve is in a closed position. The shutoff valve is mounted on a rotary shaft in the air outlet, and the flow straightener is mounted on the same rotary shaft with the shutoff valve. Crockett provides an apparatus with flow straightening vanes in order to more effectively direct a fluid flow. Crockett does not appreciate any type of vortex flow, and furthermore, does not disclose any new means of flow straightening.
Cavanagh U.S. Pat. No. 6,151,882 is directed to a turbofan engine construction having a stator portion coupled to and centrally disposed within a rigid casing. A rotor portion is disposed between the rigid casing and the stator portion for rotation about the stator portion. A portion of fluids entering the turbofan are heated between the stator and rotor portions prior to expulsion. The remainder of the fluids entering the turbofan pass unheated through the stator portion prior to expulsion. As a result, the heated fluids are expelled annularly about the unheated fluids. The result is a turbofan engine whose exhaust temperature is reduced. The inventor contemplates the invention""s utility to lie in hovercrafts since the reduced temperature of the exhaust will be able to inflate the skirt without burning it. While directed to improved propulsion for a hovercraft, Cavanagh does not propose novel flow straightening means. Further, Cavanagh does not appreciate the effect of cylindrical vortices in hovercraft performance.
Golm et al U.S. Pat. No. 6,206,635 teaches a stator device for an axial flow fan that has two rows of stator elements. The first row of stator elements is disposed axially rearward of the second row of stator elements so as to define an air flow slot therebetween. The object of the stator device is to remove rotational components from the air flow to provide a less turbulent stream, and thereby extract similar cooling power from a smaller fan motor. Golm et al, however, do not utilize contra-rotating members to eliminate rotational components, nor are vortex flows appreciated.
All systems (particularly, but not exclusively, prior ones disclosed for the first time by the inventor) that utilize a toroidal vortex to develop lift or attraction have comprised flow straightening vanes to prevent formation of parasitic cylindrical vortices. However, there are other means to this end, particularly, a contra-rotating propeller, impeller, or fan. Such an arrangement is ideal for a lifting platform, and finds its preferred embodiment therein. However, its use could be found in any of the toroidal vortex applications mentioned herein.
In short, the flow straightener could comprise a motor, or other drive means, coupled to an appropriately geared axle such that two impellers, propellers, or fans coupled thereto could rotate in opposite directions. This would simplify a lifting platform by eliminating the channel containing flow straightening vanes. A shroud could still be employed for safety, and a dihedral could be added for stability during lift.
Thus, it is an object of the present invention to eliminate cylindrical vortices in applications wherein they are undesirable.
Furthermore, it is an object of the present invention to provide a space-efficient means of straightening a flow.
It is yet another object of the present invention to provide a space-efficient means of eliminating cylindrical vortices.
It an additional object of the present invention to provide improved means for a lifting platform.
It is a further object of the present invention to provide improved means for apparatus utilizing toroidal vortices.